While Mrs Clinton declined to apologise, she said "I certainly wish I had made a different choice and I understand why the American people have questions about it".

She conceded that it was unwise to use the same private email account for both private and work-related correspondence, but insisted that doing so had been "fully above board".

Until recently Mrs Clinton had argued that the furore over her emails was merely a media obsession that did not concern everyday voters. She even made jokes about wiping the private server that housed the emails which she kept in her home in New York.

The FBI has now seized that server out of the fear that foreign governments could have had access to Mrs Clinton's correspondence.

In Friday's interview with MSNBC, the cable news channel, Mrs Clinton's tone was contrite as she conceded that the controversy would not go away any time soon.

"At the end of the day, I am sorry that this has been confusing to people and has raised a lot of questions, but there are answers to all these questions," she said.

As to recent polls showing people increasingly find her untrustworthy, Mrs Clinton said "it certainly doesn't make me feel good, but I am very confident that by the time this campaign has run its course people will know that what I've been saying is accurate."

As Mrs Clinton's campaign has laboured under the weight of the email controversy, some Democrats have begun to seek an alternative candidate.

Joe Biden, the US vice-president, has been the subject of constant speculation as he has flirted with a belated entrance into the race.

Bernie Sanders, the left-wing Vermont senator, is currently running second and has drawn massive crowds as he has travelled the country decrying systemic corruption and income inequality.

Mrs Clinton remains the prohibitive favourite to secure the nomination, but has struggled to fight on two fronts, competing for the Democratic nomination while deflecting constant attacks from Republicans.